1. Field of the Invention
This invention relates generally to a pump configured to transfer blood at a relatively low pressure and high volume, and, more particularly, to a pump configured to pump human blood while avoiding damage to the components of the blood, such as red corpuscles and platelets.
2. Description of Related Art
There have been various attempts to design a pump to replace the functions of the human heart. The attempts to date have been marred by a considerable number of problems, due in large part to the complicated nature of the human heart, as well as the tendency of blood components to be damaged when they come in contact with mechanical components under certain conditions of shear or stress.
While the most obvious replacement for a human heart would involve an emulation of the mechanical features of the human heart, previous attempts at such a solution have met with problems. The relatively large number of moving parts and reciprocating action of artificial hearts, such as the JARVIK heart, have contributed to mechanical complexity and unreliability and the configurations have made the creation of a small replaceable artificial heart difficult.
One alternative approach to direct emulation of the human heart has been to propose rotational pumps in order to gain the benefits of small size and mechanical simplicity. However, such pumps have demonstrated a tendency to damage the blood, resulting in fibrin accumulation and hemolysis, which makes the pumps unsuitable, particularly if a rotational pump is to be implanted as an artifical hart for long term use.
One approach chosen in the past has been to model the heart by designing a pump that mimics the kinematics of the heart, which is similar to the kinematics of a positive displacement pump. In a positive displacement pump, fluid is moved in discrete quantities, necessitating the use of some form of check valve to separate the quantities of fluid as they are moved.
While mechanical positive displacement pumps can mimic the human heart, they are by no means the ideal mechanical heart. Positive displacement pumps are typically bulky, complicated, and require artificial valves. It is also difficult to recreate the heart""s reciprocating motion. In addition, there are often many hidden recesses in positive displacement pumps where blood can stagnate. Blood damage due to stagnation and turbulence also typically occurs during the beginning and the end of the strokes of fixed displacement blood pumps, as the piston or diaphragm reverses direction. Centrifugal pumps have also been designed to pump blood, and have inherent advantages over positive displacement blood pumps, but in practice have also shown a tendency to damage the blood during prolonged use.
The human heart also maintains a variable output pressure and flow schedule in the presence of a varying resistance caused by vasodilation and vasocontraction of the arteries as a result of changing demand for oxygen. In practice, this requires that a pump designed to mimic the heart have some form of feedback to control the speed of the pump and regulate the volume of fluid being pumped, so that a change in the back pressure results in a change in the speed of the pump. In the human heart, this speed control is automatically governed by the central nervous system in the form of changes to the heartbeat.
Another important factor in designing an artificial heart to pump blood is the potential for destruction of blood components when blood comes in contact with mechanical surfaces. Specifically, it is known that damage to blood components such as erythrocytes (red blood cells) and platelets in a mechanical pump is a function of the stress on the blood flow stream and the time the blood is exposed to the inside of the pump. These two factors are herein referred to as the xe2x80x9cstress-time product.xe2x80x9d Each factor in turn depends on a number of variables, and those factors form design parameters in the design of a blood pump. Typically, stress imparted to a fluid flow stream is a function of the velocity of the surface in the pump that is driving the fluid, be it a rotor vane, piston face, or the walls of a contracting chamber. The faster the velocity of the pump driving surface, the greater the stress imparted to the fluid, and the greater the potential for surface-induced damage to the blood components. In conventional pumps, in order to slow down the velocity of the pump driving surface, without changing the amount of volume-flow that can be pumped by the pump, it is necessary to increase the area of the pump driving surface. But increasing the area also increases the size of the pump, which is a drawback to be avoided, especially if the pump is to be implanted.
The time and character of exposure of blood inside a pump as exemplified by a stress-time parameter are also critical issues in designing a heart pump. Excessive exposure time of blood to stress such as shear inside the pump can cause coagulation, emboli and fibrin accumulation, the latter manifesting itself in string-like particulate matter forming on the pump surfaces. In addition, flow separation, cavitation and swirl of blood streamlines can produce-undesirable thrombus material.
The flow in a centrifugal pump, unlike the flow in a positive-displacement pump, can be continuous and non-pulsating. This results in a lower maximum velocity, and consequently the stress imparted by the pump surfaces on the blood is lower. Tests have shown that the maximum velocity in a centrifugal-type blood pump can be significantly less than the maximum velocity in a positive displacement-type blood pump. Animal tests have also shown acceptable longtime performance with steady state flow. However, some researchers feel that certain organs such as the kidneys may be affected by nonpulsing flow.
Thus, there remains a need for a compact, reliable, and simple blood pump that may be used to temporarily or permanently replace a defective human heart. It would be advantageous if such a blood pump was easy to manufacture, placed relatively little stress upon the blood components being pumped, and was easily driven by an external power source. The present invention satisfies all of these requirements.
Briefly, and in general terms, the present invention provides for an improved centrifugal blood pump apparatus that can replace some or all of the function of the human heart, either as a xe2x80x9cbridge pumpxe2x80x9d for use from a few days to several weeks in anticipation of a heart transplant operation, a ventricular assist device (VAD) for extracorporeal use during open-heart surgery, or, with suitable control systems, as a total artificial heart (TAH). The present invention utilizes a variety of control systems to maintain a stable, dual fluid circuit, centrifugal blood pump apparatus. Employed as part of this apparatus is a new and improved centrifugal blood pump that does not suffer from many of the disadvantages of positive displacement blood pumps and conventional centrifugal pumps. The centrifugal blood pump of the invention is lightweight, small enough to be implantable, and simple in operation. The centrifugal blood pump of the invention provides for controlled flow in the presence of fluctuations in pressure, and eliminates areas of stagnant flow. The geometry of the centrifugal blood pump also minimizes the stress-time product on the blood being pumped.
In another aspect of the present invention, a blood pump apparatus is provided utilizing centrifugal pumps to serve as an artificial heart. Despite the numerous advantages a centrifugal pump has over a positive displacement pump for use as an artificial heart, some form of rotational speed control to vary the pump speed is required to maintain a variable output pressure and flow schedule in the presence. of a varying resistance of the arteries. The invention accordingly provides for proper speed control of centrifugal blood pumps, so that a life support system that utilizes such pumps can maintain the proper operational characteristics required by the cardiovascular system attached to and sustained by the life support system. Thus the difficulty in maintaining a programmed flow rate in a fluid circuit, when using centrifugal pumps that experience changes in their outlet pressure, is reduced. By use of the invention, dual centrifugal pumps may be used to pump blood throughout a human cardiovascular system, and the benefits in using such centrifugal blood pumps in lieu of positive displacement pumps are realized.
In one presently preferred embodiment of the invention, a centrifugal blood pump comprises a pump enclosure having an inlet and an discharge outlet, the inlet disposed along a longitudinal axis; and an impeller surrounded by the enclosure, comprising a disc, rotatable about and radially extending from the longitudinal axis, the disc having a front. face coaxial with the inlet and a back face facing away from the front face, and a plurality of blades attached to the front face of the disc. The housing comprises a stationary shroud adjacent to the blades. A primary fluid passageway is also provided between the stationary shroud and the front face of the disc and a secondary fluid passageway between the back face of the disc and the enclosure; and an opening is provided in the disc allowing fluid communication between the primary fluid passageway and the secondary fluid passageway, the opening allowing fluid to flow between the primary and secondary fluid passageways, to prevent stagnation of fluid flow.
In another preferred embodiment, the invention provides for a dual pump artificial heart blood pump apparatus for use in a mammalian cardiovascular system comprising a first pump, having an input and an output connectable to the cardiovascular system; means for sensing pressure at the output of the first pump; means for sensing pressure at the input of the first pump; and means for sensing flow rate through the first pump. A second pump is also provided, having an input and an output connectable to the cardiovascular system; means for sensing pressure at the output of the second pump; means for sensing pressure at the input of the second pump; means for sensing flow rate through the second pump; and means for controlling the speed of the first and second pumps based on the pressures and flow rates detected by the sensing means.
The present invention also provides for a method of controlling the operation of a centrifugal blood pump having an input and an output connected to a cardiovascular system, the method comprising the steps of sensing a pressure in the cardiovascular system and generating a pressure signal indicative of the pressure in the cardiovascular system; and controlling the speed of operation of the centrifugal blood pump responsive to the pressure signal. In another preferred embodiment, the invention provides for a method of controlling the operation of a centrifugal blood pump having an input and an output connected to a cardiovascular system, the method comprising the steps of sensing a blood flow rate in the cardiovascular system and generating a blood flow rate signal indicative of the blood flow rate in the cardiovascular system; and controlling the speed of operation of the centrifugal blood pump responsive to the blood flow rate signal.
Another presently preferred embodiment of the invention provides for a method of controlling dual centrifugal blood pumps operating as a heart blood pump, with a first centrifugal blood pump having an inlet connected to a cardiovascular system for receiving blood from the cardiovascular system and providing output to a respiratory system, and a second centrifugal blood pump having an inlet connected to the respiratory system to receive blood from the respiratory system and connected to deliver output to the cardiovascular system. The method comprises the steps of measuring a first pressure at the inlet of the first centrifugal blood pump and generating a first pressure signal indicative of the first pressure; controlling the speed of operation of the second centrifugal blood pump responsive to the first pressure signal; measuring a second pressure at the inlet of the second centrifugal blood pump and generating a second pressure signal indicative of the second pressure; and controlling the speed of operation of the first centrifugal blood pump responsive to the second pressure signal. Temperature and pressure sensing stations described above may be omitted or augmented and used in different sequences without violating the principles of the invention.
In yet another presently preferred embodiment, the invention provides a method of controlling the operation of a centrifugal blood pump having an input and an output connected to a cardiovascular system, the steps of the method comprising providing a pulse frequency for periodically changing blood pressure in the cardiovascular system; and controlling the speed of operation of the centrifugal blood pump responsive to the pulse frequency for periodically changing the blood pressure.
In another aspect of a presently preferred embodiment of the invention, the invention provides a system for connecting a centrifugal blood pump having an inlet and an output adapted to be connected to a mammalian cardiovascular system, comprising a flexible pump inlet cuff shaped in the form of a zone of a sphere, the inlet cuff having a surface defining a first opening and a second opening, the first opening being smaller than the second opening. Also provided are means for connecting the first opening in the inlet cuff to the inlet of the pump; means for connecting the second opening in the inlet cuff to the left atrium of a mammalian heart; and means for connecting the second opening in the inlet cuff to the right atrium of a mammalian heart.
Another presently preferred embodiment of the invention provides a method of connecting an artificial blood pump including a left ventricular blood pump and a right ventricular blood pump, the artificial blood pump having an inlet and an outlet adapted to be connected to a mammalian cardiovascular system. The method comprises separating the left ventricle from the left atrium below the aortic valve and the mitral valve; separating the right ventricle from the right atrium below the atrioventricular valve and the pulmonary valve; connecting the centrifugal blood pump and an inlet cuff with sutures to the mammalian heart; connecting an outlet of the left ventricular blood pump to biocompatible tubing and the tubing with sutures to the ascending aorta; and connecting an outlet of the right ventricular blood pump to biocompatible tubing and the tubing with sutures to the pulmonary artery.
The rotor blades of the impeller of the centrifugal pump utilized in the invention are designed to have a diameter such that their tip velocity does not create stress on the blood exceeding the maximum stress allowed by the stress-time product design parameter. Furthermore, the inlet diameter, stationary shroud, blade shroud contour, torus of the stationary shroud and the clearances between moving parts of the centrifugal pump are designed to minimize excessive stress on the blood. In addition, the geometry of the pump is such that areas in the pump that will cause stagnation, separation, or swirl of the blood streamlines, are minimized. In the present invention, the hydrodynamic efficiency of the pump is deliberately reduced in order to minimize any stagnant or recirculating flow. The invention also provides for a method of mild pulsation of blood being pumped by the centrifugal blood pump apparatus of the invention. These and other features of the present centrifugal blood pump apparatus and method of the invention are designed to minimize the stress-time product on the blood, as will be explained below.